The present invention relates to a magnetic recording medium having a ferromagnetic thin layer serving as a magnetic layer, and more particularly to a magnetic recording medium satisfying requirements for still characteristics and durability as well as high electromagnetic characteristics.
With recent increases in recording density of magnetic recording media, the thickness of the recording media has been decreased. The still characteristics and the durability are deteriorated and the reproduced output in a low density region is lowered with decreasing film thickness. Various magnetic recording media have hitherto been proposed to improve the electromagnetic characteristics, the still characteristics, the durability and the corrosion resistance. For example, Japanese Patent Unexamined Publication No. 54-145505 discloses a magnetic recording medium comprising a polymer molding serving as a substrate; a deposit layer formed on a surface of the substrate, said deposit layer being formed of any one element selected from the group consisting of gold, silver and copper; a deposit layer formed thereon, said deposit layer being formed of a material having a high affinity for oxygen; and a magnetic layer further formed thereon. Japanese Patent Unexamined Publication No. 1-124115 discloses a magnetic recording medium comprising a substrate, an undercoat layer formed thereon and a magnetic layer formed on said undercoat layer by oblique deposition of a ferromagnetic metal material, in which said undercoat layer is formed of a metal or an alloy containing at least one element selected from the group consisting of aluminum, chromium, titanium and copper, and said magnetic layer is formed of the ferromagnetic metal material composed of cobalt or mainly composed thereof and comprises an aggregate of columnar magnetic particles inclined to the normal line of said substrate. Japanese Patent Unexamined Publication No. 2-68712 discloses a thin film type magnetic recording medium comprising a non-magnetic substrate, a non-magnetic undercoat layer formed thereon by paper deposition of a material having a Mohs' hardness of 5 or more, and a recording magnetic layer formed on the undercoat layer through or not through a soft magnetic layer. Japanese Patent Unexamined Publication No. 2-179917 discloses a magnetic recording medium comprising a non-magnetic substrate and a ferromagnetic metal thin film formed thereon, in which an undercoat layer formed of titanium is provided between said non-magnetic substrate and said ferromagnetic metal thin film, and the ferromagnetic metal thin film is laminated with a titanium layer and a diamond-like hard carbon coating layer. Japanese Patent Unexamined Publication No. 61-294635 discloses a magnetic recording medium comprising a polymer film substrate, a Co oxide film formed thereon by thin film deposition and a magnetic recording layer of a ferromagnetic metal further formed thereon by thin film deposition. Japanese Patent Unexamined Publication No. 1-208720 discloses a magnetic recording medium comprises a non-magnetic substrate and multiple deposit magnetic layers different in oxygen content laminated thereon. Japanese Patent Unexamined Publication No. 4-26914 discloses a magnetic recording medium comprising a non-magnetic substrate and a magnetic layer formed thereon by oblique deposition, said magnetic layer being composed of at least two ferromagnetic metal thin films, said ferromagnetic metal thin films containing Co--Ni or Co--Ni--Cr as a main component, in which the Co content of the ferromagnetic metal thin film of the uppermost layer is lower than that of the ferromagnetic metal thin film of the lowest layer.
However, in the prior-art magnetic recording media such as the magnetic recording media described in Japanese Patent Unexamined Publication Nos. 54-145505, 1-124115, 2-68712 and 2-179917, the undercoat layers are provided between the non-magnetic substrates and the ferromagnetic thin films to improve the still characteristics, the durability and the corrosion resistance, thereby attempting to enhance adhesion and strength of the magnetic films, and the use of the non-magnetic films under the ferromagnetic thin films lowers the reproduced output at a low density.
Furthermore, in the magnetic recording media described in Japanese Patent Unexamined Publication Nos. 61-294635, 1-208720 and 4-26914, attempts are made to improve the still characteristics, the durability and the reproduced output over the range of a low density to a high density by the use of the ferromagnetic thin films in multiple layers. However, the multiple layers causes the output over the range of a low density to a high density to be lowered, compared with the single thin layer.
Accordingly, a magnetic recording medium has been desired in which the reproduced output over the range of a low density to a high density are not lowered, and in which the still characteristics and the durability can be satisfied.